1.) Field of the Invention
The present invention relates to a method and apparatus for determining a high temperature state of an air-fuel ratio sensor, such as a titania-type O.sub.2 sensor, in an internal combustion engine.
2.) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
As the above-mentioned O.sub.2 sensor, a titania (TiO.sub.2) type O.sub.2 sensor having a high response characteristic is used. Namely, the element resistance of the titania O.sub.2 sensor is small when the air-fuel ratio is rich, and is large when the air-fuel ratio is lean. The element resistance of the titania type O.sub.2 sensor, however, is affected strongly by the temperature thereof, compared with zirconia type O.sub.2 sensors; i.e., when the temperature of the titania type O.sub.2 sensor is increased, an output thereof indicating a lean state is close to that indicating a rich state, and as a result, when the above-mentioned air-fuel ratio feedback control is carried out, the controlled air-fuel ratio may be overlean, thus increasing NO.sub.x emissions, and inviting knocking, misfiring, and the like. Therefore, it is important to detect a high temperature state of the titania type O.sub.2 sensor. Note, such a high temperature state can be detected by incorporating a temperature sensor but this increases the manufacturing cost. In the prior art, such a high temperature state is detected by determining whether or not an extreme value, such as a minimum value, of the output of the titania type O.sub.2 sensor is higher than a predetermined value (see Japanese Patent Publication Nos. 57-105529 and 57-143143).
In the above-mentioned prior art, however, even when the temperature of the titania type O.sub.2 sensor is actually low, a high temperature state thereof is erroneously determined, as later explained in more detail.